gfs2: Turn gfs2_extent_map into gfs2_{get,alloc}_extent

Convert gfs2_extent_map to iomap and split it into gfs2_get_extent and
gfs2_alloc_extent.  Instead of hardcoding the extent size, pass it in
via the extlen parameter.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>